1. Field of the Invention
The present invention relates to a fixer device for a thermal printer. More particularly, the present invention relates to a fixer device for a thermal printer, in which ray emitting intensity of a fixing ultraviolet lamp can be regulated for stable fixation of an optically fixable coloring layer, even after long use of the fixing lamp.
2. Description Related to the Prior Art
A thermal printer has a thermal head, and is used with thermosensitive recording paper. The thermosensitive recording paper includes a support and at least one thermosensitive coloring layer, which is pressed and heated by the thermal head for recording an image. Then ultraviolet rays are applied to the recording paper to fix the image. There also exists color thermosensitive recording paper, which includes a support and cyan, magenta and yellow thermosensitive coloring layers.
The yellow coloring layer is overlaid the closest to the obverse, and has the highest thermal sensitivity. The yellow coloring layer is colored using little heat energy. The cyan coloring layer is overlaid the farthest from the obverse, and has the lowest thermal sensitivity. The cyan coloring layer is colored in cyan at a great heat energy. To print an image, a thermal head is pressed against the recording paper while the recording paper is moved. The thermal head applies heat to the recording paper, to effect the thermal recording to the respective coloring layers in the order from the yellow coloring layer toward the support. The yellow coloring layer and the magenta coloring layer are fixed when receiving application of ultraviolet rays having wavelength ranges specific to the yellow coloring layer and the magenta coloring layer respectively. During fixation, coloring components in the yellow coloring layer and the magenta coloring layer that are not colored are photochemically decomposed, to lose their coloring ability.
A yellow fixing ultraviolet lamp is driven to apply the near ultraviolet rays having an emitting peak of 420 nm to the yellow coloring layer. If the amount of ultraviolet rays applied to the yellow coloring layer is too small, a coloring component in the yellow coloring layer partially remains without optically decomposition. The yellow coloring component is colored during the recording of the magenta coloring layer. The ultraviolet rays for yellow fixation should not be applied at an amount over a suitable amount for fixing the yellow coloring layer, because there would be considerable influence of the ultraviolet rays for yellow fixation on the magenta coloring layer. Therefore the yellow fixing lamp is driven by properly adjusted energy.
The ultraviolet rays having an emitting peak of 365 nm is applied to the magenta coloring layer. If the amount of ultraviolet rays applied to the magenta coloring layer is too small, a coloring component in the magenta coloring layer partially remains without optically decomposing. The cyan coloring layer does not have photochemical fixability. It is allowable that the ultraviolet rays for magenta fixation is applied at an amount over a suitable amount for fixing the magenta coloring layer, because there is no influence of the ultraviolet rays for magenta fixation on the cyan coloring layer. Therefore a magenta fixing ultraviolet lamp is fully driven, because no adjustment of the ultraviolet rays for magenta fixation is required.
U.S. Pat. No. 5,486,856 discloses a feedback control, which is effected for the yellow fixing lamp by use of an irradiance sensor. The ray emitting intensity of the yellow fixing lamp is controlled to adjust a measured irradiance to be a target irradiance. The feedback control calculates a difference between the measured irradiance from the irradiance sensor and the target irradiance at which a suitable ray applying amount can be acquired in consideration of a conveying speed of the recording paper. The irradiance difference is multiplied by a constant gain value, to determine a correcting value. The lamp control value is corrected according to the correcting value, to set the measured irradiance equal to the target irradiance.
The fixing lamp has ray emitting intensity which decreases with time in the course of long use. In the known fixer device, the gain value is determined in a factory where the printer is manufactured and adjusted. If the ultraviolet lamp after the long time has a small ray emitting intensity, ray adjustment of the fixing lamp is slow. In printing operation, a considerable duration is required for acquiring the target irradiance. There occurs a problem in that a front portion of a recording area on a recording sheet is short of an amount of fixation.